Method of constant-current polarization voltage and apparatus for Karl-Fischer technique

ABSTRACT

A constant-current polarization voltage detecting method includes step of feeding to a detecting electrode a predetermined minute current in a pulse form, and detecting sequentially a polarization voltage at each time of feeding the pulsating current, in an electrochemical analysis in solution. The polarization voltage is detected after a lapse of a predetermined time from the initiation of feeding the pulsating current at each time. A Karl Fischer&#39;s moisture content analyzing apparatus can utilize such a method.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a constant-current polarization voltagedetecting method used suitably in a Karl Fischer's moisture contentmeasuring method and a Karl Fischer's moisture content measuringapparatus using such detecting method.

2. Discussion of the Background

For a detecting means to measure a moisture content for which a KarlFischer's (hereinbelow, referred to as “KF”) volumetric titration methodof this kind is used, a so-called constant-current polarization voltagedetecting method has often been employed in recent years. Namely, in theconstant-current polarization voltage detecting method, a minute currentis fed to dual platinum electrodes, as a detecting electrode, to measurea voltage across the dual platinum electrodes. In this case, although aminute current to be fed can either be a direct current or analternating current, a pulse-like current has recently been used.

On the other hand, when a titration solvent composed mainly ofchloroform as solvent was used for the constant-current polarizationvoltage detecting method, there was in fact difficulty in conducting themeasurement of a large amount of sample in a moisture content measuringapparatus by a conventional technique. This was because the monitoringof a polarization voltage could not properly be effected since theliquid resistance of a chloroform solvent is high, and the liquidresistance is further increased by the incorporation of a large amountof sample having no-polarization properties in the titration solvent.

The moisture content measurement using the above-mentioned KF volumetrictitration method is such one utilizing KF titration reactions asfollows:

I₂+SO₂+H₂O+3BASE→2BASE.HI+BASE.SO₃

BASE.SO₃+CH₃OH→BASE.HSO₄CH₄

wherein BASE: an amine compound

Namely, in the titration reactions in the moisture content measurementby the KF volumetric titration method, since the reaction with wateraccelerates selectively, a wide application has conventionally been madefor the measurement of moisture content. In this case, in a KFvolumetric titration method, measurement is carried out by using aniodine-containing solution as titrant. In a coulometric titrationmethod, iodine is generated by anode oxidation of iodide ions. Thus, theabove-mentioned reactions are effected. In either method of detection,the end point of titration is recognized when an excessive amount ofiodine is detected on the detecting electrode.

In such detecting method for detecting an excessive iodine, theabove-mentioned constant-current polarization voltage detecting methodis generally used, wherein according to the volumetric titration method,the titration is finished at the time when a state that an amount ofiodine is excessive (a state a potential which is lower than that at theend point) continues 30 seconds, and according to the coulometrictitration method, the titration is finished at the time when a potentialis beyond the potential at the end point at which a state of a slightiodine being excessive can be detected.

Generally, there is no problem in a conventional titration method evenwhen solvent containing much methanol is used as a titration solvent,and the titration is conducted by using a KF reagent. Even though anysample is incorporated, it is possible to reach the end point if the KFreagent is dropped excessively, and the moisture content measurement canbe conducted regularly.

However, in a case of measuring oils, the titration is usually conductedin a titration solvent containing chloroform as a main solvent becausethe oils do not dissolve in methanol. However, since oils generally havea small moisture content, a large amount of sample is to be poured intothe titration solvent. In this case, the presence of a much amount ofchloroform increases the liquid resistance of the titration solvent. Theincorporation of a much amount of sample further increases the liquidresistance. Under such condition, when a predetermined minute current ofpulse form is applied, an apparent polarization voltage is assumed to bethe sum of the true polarization voltage and a polarization voltageresulted from the liquid resistance. Accordingly, even though thetitration in fact reaches the end point of titration, there is found aphenomenon as if it does not reach the end point of titration.

SUMMARY OF THE INVENTION

The present invention is to solve the problems in the conventionaltechnique, and aims at providing a constant-current polarization voltagedetecting method for measuring a moisture content by a KF titrationreaction wherein the detection or the monitoring of a polarizationvoltage under a regular condition can be conducted, and a Karl Fischer'smoisture content measuring apparatus using such method.

In order to achieve the above-mentioned object, the inventors of thisapplication have made various studies, to improve the above-mentionedproblems, on the constant-current polarization voltage detecting methodfor measuring a moisture content in the KF titration reaction, as theresult of which they have found an important fact. Namely, influence toa polarization voltage due to solvent, when a current is applied in apulse form, is usually large immediately after the application.

Description will be made in the following as to the result of thestudies. FIG. 1 is a diagram showing a relation of time to polarizationvoltage in Karl Fischer's moisture content measurement using theconstant-current polarization voltage detecting method. In this case,determination is made so that a cycle of the application of a pulsatingminute current is 500 ms and the pulse width of the current actuallyapplied is 50 ms.

As shown in FIG. 1, when an ordinarily used sample is measured with amethanol type solvent, a polarization voltage measured on a detectingelectrode increases gradually under the condition of the moisturecontent being excessive, and it decreases when the applied current isstopped.

When a KF reagent containing iodine is dropped to titrate a moisturecontent, an increase of polarization voltage tends to gradually decreaseto thereby cause the KF reagent becoming excessive, with the result thatthe polarization voltage becomes very low. In this case, in acommercially available moisture content measuring apparatus using avolumetric method, an end point of analysis is determined when apolarization voltage, which has been detected as it is, takes apredetermined value (i.e., a value at the time when the KF reagentbecomes excessive. This expression is applicable hereinbelow). Or, theend point is so determined that a waveform of a polarization voltage ina time is detected; an integration value of the polarization voltage ina one cycle (the surface area of a hatched portion in FIG. 1) isdetected, and the integration value reaches a predetermined value atwhich the end point is determined.

On the other hand, in the measurement of a moisture content in oils,since oils are insoluble to methanol, solvent containing chloroform asthe main solvent is used. As described above, however, since the solventcontaining chloroform as the main solvent has a high liquid resistance,a detected voltage is the sum of a voltage due to the liquid resistanceand a true polarization voltage. Further, when a sample is added to thesolvent, the influence of the liquid resistance becomes remarkable. Asclear from FIG. 1, it is understood that the polarization voltage showsan abnormal pattern just after the application of a pulsating current.Although the dropping of the KF reagent to titrate a moisture contentreduces somewhat the polarization voltage, a value lower than a certainvalue can not be expected. As a result, although the moisture content inthe sample has been able to be titrated by the KF reagent, the droppingof the KF reagent is continued because a polarization voltage does notdecrease to a voltage at the end point (or an integration value ofpolarization voltage).

In consideration of these circumstances of the measurement of moisturecontent by the KF titration reaction using the above-mentionedconstant-current polarization voltage detecting method, the inventorshave made effort to eliminate an adverse effect to a polarizationvoltage due to the solvent, and have completed the present invention byremoving a polarization voltage produced just after the application of apulsating minute current whereby a polarization voltage close to thetrue value can be detected.

Namely, in accordance with a first aspect of the present invention,there is provided a constant-current polarization voltage detectingmethod comprising feeding to a detecting electrode a predeterminedminute current in a pulse form, and detecting sequentially apolarization voltage at each time of feeding the pulsating current, inan electrochemical analysis in solution, the constant-currentpolarization voltage detecting method being characterized in that thepolarization voltage is detected after a lapse of a predetermined time(to) from the initiation of feeding the pulsating current at each time.

According to a second aspect, there is provided a constant-currentpolarization voltage detecting method comprising feeding to a detectingelectrode a predetermined minute current in a pulse form, and detectingsequentially a polarization voltage at each time of feeding thepulsating current, in a Karl Fischer's moisture content analysis, theconstant-current polarization voltage detecting method beingcharacterized in that a polarization voltage difference (X−Xo) obtainedby subtracting a polarization voltage (Xo) after a lapse of apredetermined time (to) from the initiation of feeding the pulsatingcurrent at each time, from a polarization voltage (X) at each time offeeding the pulsating current, is detected.

According a third aspect, there is provided a Karl Fischer's moisturecontent measuring apparatus using a constant-current polarizationvoltage detecting method, the Karl Fischer's moisture content measuringapparatus being characterized by comprising means for feeding a constantcurrent in a pulse form to a detecting electrode immersed in a reactionliquid, means for detecting sequentially a polarization voltage (X)during the feeding of the current, means for calculating a value (X−Xo)obtained by subtracting a polarization voltage (Xo) after a lapse oftime (to) from the initiation of feeding the pulsating current at eachtime, from a polarization voltage (X) at each time of feeding thepulsating current detected sequentially, means for determining the endpoint of measurement of moisture content by using the polarizationvoltage difference (X−Xo), and means for calculating a moisture contentconcentration from a result of calculation.

Hereinbelow, description will be made in detail as to theconstant-current polarization voltage detecting method and the KarlFischer's moisture content measuring apparatus according to the presentinvention.

The Karl Fischer's (hereinbelow, referred to as “KF”) moisture contentmeasuring apparatus used in the present invention is shown in, forexample, FIG. 3.

The apparatus comprises a titration section and a measuring/displayingsection wherein the titration section comprises a titration flask and aKF reagent titration device, and the measuring/displaying sectioncomprises a detecting section, a data processing section and a controlsection.

A KF reactive solvent is received in the titration flask. The KFreactive solvent can be used without particular limitation as far as itcan be used for an ordinary KF moisture content measurement. However,when a reactive solvent having a high liquid resistance such as achloroform series solvent or the like is used, the effect of the presentinvention is in particular remarkable. For example, the presentinvention provides an excellent effect in a case that a KF reactivesolvent containing chloroform of more than 30% is used. A detectingelectrode is immersed in the reactive solvent. A minute current ofpulse-like form is applied to the detecting electrode. The applicationof the pulsating current means that, as shown in FIG. 2, an applicablecycle is formed of a predetermined time of current feeding and apredetermined time of current stopping, and the cycle is repeated. Theapplicable cycle is preferably 100-3,000 ms, more preferably, 300-700ms. The pulse width of the current fed to the defecting electrode ispreferably 10-1,000 ms, more preferably 10-100 ms. An applicable currentis preferably 3-100 μA, more preferably, 3-30 μA. A sample to bemeasured is poured through the sample introducing port of the titrationflask when the moisture content is to be measured.

After the introduction of the sample to be measured into the titrationflask, a KF titration reagent including iodine is added with constantintervals from the KF reagent titration device into the titration flask.An amount of the titration reagent to be added in a time is controlledby the control section from which a signal is transmitted so that apulse motor drives a piston buret, whereby the KF titration reagent isforced from the KF reagent titration device and is dropped through thetitration nozzle. As a moisture content in the titration flask islarger, a much amount of the titration reagent is added. The addition ofthe titration reagent is conducted when the current is not applied tothe detecting electrode.

FIG. 2 is a diagram showing a relation of time to polarization voltagein order to explain the detecting method of the present invention.

The detecting section has a mechanism for applying a minute current tothe detecting electrode immersed in the KF reactive solvent so that apolarization voltage (X) across electrodes in the detecting electrode isdetected.

The data processing section calculates a value (X−Xo) which is obtainedby subtracting a polarization voltage (Xo) after a lapse of apredetermined time (to) from the initiation of feeding the pulsatingcurrent at each time, from a value of polarization voltage (X) at eachtime of feeding the pulsating current detected sequentially. After alapse of a predetermined time (to) from the initiation of feeding thecurrent indicates the time at which an abnormal disturbance in thepolarization voltage produced just after the application of a pulsatingcurrent to the detecting electrode has disappeared. For example, in viewof a polarization voltage curve obtained by measuring a moisture contentin an electrical insulation oil by using a highly chloroform-containingsolvent in FIG. 2, the polarization voltage once drops just after theapplication of the current and then, increases. A predetermined time(to) indicates the time at which the polarization voltage begins risingto depict a smooth polarization voltage curve. Specifically, thepredetermined time (to) from the initiation of feeding the current,which varies depending on conditions of measurement, is generally0.1-200 ms, preferably, 0.1-50 ms, more preferably, 0.1-5 ms. Forexample, a polarization voltage is previously measured before theinitiation of titration; the time at which a smooth polarization voltagecurve is to be depicted is determined as “to”, and the value of “to” ispreviously stored in a KF analyzing device. When a methanol seriessolvent is used, the value of “to” is optional. However, when a value of“to” is large, a value of polarization voltage difference (X−Xo)detected becomes small. Accordingly, it is preferable that the value of“to” is small from the viewpoint of accuracy. When a value of “to”suitable for using a highly chloroform-containing solvent is previouslymemorized in the analyzing device, the measurement of a moisture contentcan accurately be conducted with the same device even in a case of usingthe highly chloroform-containing solvent or a case of using a methanolseries solvent.

Then, an integration value of polarization voltage difference (X−Xo) iscalculated for every one cycle of feeding after a lapse of apredetermined time (to) from the initiation of feeding the current. Theintegration value is expressed by a hatched portion in the waveform of apolarization voltage in FIG. 2. Depending on the magnitude of theintegration value in one cycle, an amount of the reagent for titrationfrom the KF reagent titration device is controlled by the controlsection. When the integration value reaches a previously determinedvalue (a value in a state that the KF titration reagent becomesexcessive), the end point of analysis is determined, and the dropping ofthe reagent is stopped. The predetermined value is previously inputtedinto the measuring device by measuring separately a value in a statethat the KF is excessive. As to the way of determining the predeterminedvalue, for example, a methanol series solvent having a low liquidresistance is used as the KF reactive solvent, and an integration valuein a state that the KF titration reagent is excessive is measured.

After the completion of the titration, a moisture content in a sample tobe measured is calculated in the data processing section based on anamount of the KF titration reagent used, and a calculated value isoutputted.

The above-mentioned end point determining method is a method todetermine the end point with use of an integration value of polarizationvoltage difference. However, the end point may be determined when thepolarization voltage difference (X−Xo) reaches a predetermined value.However, the way of determination with an integration value is preferredbecause of improving accuracy in analysis.

Further, description has been made exemplifying the volumetric titrationmethod. However, the present invention is usable for a coulometrictitration method, and an analyzing method using the coulometrictitration method and an apparatus using the same are included in thescope of the present invention as far as they are not beyond thedescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of a polarization voltage curvein a case that a current is fed in a pulse form to a detecting electrodein a constant-current polarization voltage detecting method whereinintegrated portions monitored by a conventional moisture content endpoint detecting method are shown.

FIG. 2 is a diagram showing an example of a polarization voltage curvein a case that a current is fed in a pulse form to the detectingelectrode in the constant-current polarization voltage detecting methodwherein integrated portions monitored according to the present inventionare shown.

FIG. 3 is a diagram showing an example of a KF analyzing device used inthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXPERIMENTAL EXAMPLE 1

A moisture content in an electrical insulation oil was measured with useof the KF moisture content measuring device shown in FIG. 3. First, 50ml of a dehydrated solvent CM (manufactured by Mitsubishi ChemicalCorporation, the content of chloroform: 87%) was charged into atitration flask, and a previous titration was conducted with KF reagentSS 3 mg (manufactured by Mitsubishi Chemical Corporation) to removewater in the titration flask. Then, each amount of 20 ml of theelectrical insulation oil was measured. A current of 25 μA was appliedto the detecting electrode wherein a cycle of the application of thecurrent was 500 ms and a pulse width of the applied current was 25 ms. Apolarization voltage (X) during the feeding of the current wassequentially detected, and a value (X−Xo) obtained by subtracting apolarization voltage (Xo) after a lapse of 1 ms from the initiation offeeding the pulsating current at each time, from the polarizationvoltage (X) during the feeding of the current detected sequentially, wascalculated. The end point of analysis was determined at the time when anintegration value (the surface area of a hatched portion in FIG. 2) ofthe polarization voltage difference (X−Xo), in one cycle of currentfeeding, after a lapse of 1 ms from the initiation of feeding thecurrent reached a predetermined value (a value in a state that the KFreagent became excessive), and then, a moisture content in theelectrical insulation oil was calculated.

COMPARATIVE EXAMPLE 1

The measurement was conducted in the same manner as Example 1 exceptthat the end point of analysis was determined at the time when anintegration value (the surface area of a hatched portion in FIG. 1), ina one time of current feeding, of the polarization voltage (X) from theinitiation of feeding the current reached a predetermined value (a valuein a state that the KF reagent became excessive).

Results of Example 1 and Comparative Example 1 are shown in Table 1.

TABLE 1 Result of measurement Quantity Moisture Analyzed of samplecontent Time value Moisture content 17.3145 g 0.604 mg 1′54″ 35 ppmmeasuring apparatus 17.1783 g 0.572 mg 1′18″ 33 ppm of the present17.2235 g 0.527 mg 1′40″ 31 ppm invention Av: 33 ppm RSD: 6.6% Moisturecontent 17.0075g It does not reach the end point measuring apparatus ofa conventional example

EXPERIMENTAL EXAMPLE 2

50 ml of dehydrated solvent OLII (manufactured by Mitsubishi ChemicalCorporation, the content of chloroform: 82%) was charged into atitration flask, and a previous titration was conducted with KF reagentSS-X 3 mg (manufactured by Mitsubishi Chemical Corporation) to removewater in the titration flask. Then, each amount of 10 ml of Kerosene wasmeasured. The other conditions for measurement were the same as those inExample 1.

COMPARATIVE EXAMPLE 2

The measurement was conducted in the same manner as Example 2 exceptthat the conventional end point determining method described inComparative Example 1 was used. Results of Example 2 and ComparativeExample 2 are shown in Table 2.

TABLE 2 Result of measurement Quantity Moisture Analyzed of samplecontent Time value Moisture content 7.7091 g 0.348 mg 0′56″ 45 ppmmeasuring apparatus 7.9134 g 0.388 mg 1′04″ 49 ppm of the present 7.3733g 0.403 mg 1′37″ 55 ppm invention 7.6924 g 0.336 mg 1′05″ 44 ppm 7.6784g 0.307 mg 1′34″ 40 ppm Av: 47 ppm RSD: 12% Moisture content 7.8415 g0.336 mg 0′37″ 43 ppm measuring apparatus 7.6985 g 0.552 mg 0′41″ 72 ppmof a conventional 7.7132 g It does not reach the end point example

As is clear from the above experimental Examples 1 and 2, the KFtitration was normally progressed even by using the solvent containingchloroform as the major solvent, and a correct moisture contentmeasurement of a sample containing a much amount of oil, for which themeasurement was impossible, became possible.

As described in detail, according to the constant-current polarizationvoltage detecting method and the apparatus using the method in thepresent invention, the monitoring of a polarization voltage can beconducted wherein influence to the polarization voltage due to solventcan effectively and sufficiently be eliminated, whereby the measurementof moisture content can effectively and correctly be conducted. Further,there is an excellent feature that the method can easily be conductedsince the above-mentioned method itself is very simple.

What is claimed is:
 1. A constant-current polarization voltage detectingmethod for use in a Karl-Fischer's moisture content analysis,comprising: (a) delivering a predetermined pulse current including atleast one pulse to a detecting electrode; (b) detecting a firstpolarization voltage after a lapse of a predetermined time from the timeof initiating the delivery of the at least one pulse of the pulsecurrent; (c) detecting a second polarization voltage at a timesubsequent to the detection of the first polarization voltage; and (d)obtaining a polarization voltage difference by subtracting the detectedfirst polarization voltage from the detected second polarizationvoltage.
 2. A constant-current polarization voltage detecting methodaccording to claim 1, further comprising: integrating the polarizationvoltage difference during one cycle of current delivery and obtaining anintegration value; ending measurement of the moisture content when saidintegration value of the polarization voltage difference reaches apredetermined value.
 3. A Karl Fischer's moisture content measuringapparatus comprising: (a) means for delivering a constant current in apulse form including at least one pulse to a detecting electrodeimmersed in a reaction liquid; (b) means for detecting a firstpolarization value after a lapse of a predetermined time from the timeof initiating the delivery of the at least one pulse of the pulsecurrent; (c) means for detecting a second polarization voltage at a timesubsequent to the detection of the first polarization voltage; (d) meansfor calculating a polarization voltage difference value by subtractingthe detected second polarization voltage from the detected firstpolarization voltage; (e) means for determining an end point ofmeasurement of moisture content based on the calculated polarizationvoltage difference value; and (f) means for calculating a moisturecontent concentration from a result of the means for calculating.
 4. AKarl-Fischer's moisture content measuring apparatus according to claim3, wherein an end point of measurement of the moisture content is a timeat which an integration of the polarization difference value in onecycle of current delivery, reaches a predetermined value.
 5. A KarlFischer's moisture content measuring apparatus according to claim 4,wherein said predetermined time is 0.1-200 ms.
 6. A Karl Fischer'smoisture content measuring apparatus according to claim 5, wherein theKarl Fischer's moisture content measuring apparatus is used involumetric titration.
 7. A Karl Fischer's moisture content measuringapparatus according to claim 5, wherein the Karl Fischer's moisturecontent measuring apparatus is used in coulometric titration.
 8. A KarlFischer's moisture content measuring apparatus according to claim 4,wherein the Karl Fischer's moisture content measuring apparatus is usedin volumetric titration.
 9. A Karl Fischer's moisture content measuringapparatus according to claim 4, wherein the Karl Fischer's moisturecontent measuring apparatus is used in coulometric titration.
 10. A KarlFischer's moisture content measuring apparatus according to claim 3,wherein said predetermined time is 0.1-200 ms.
 11. A Karl Fischer'smoisture content measuring apparatus according to claim 10, wherein theKarl Fischer's moisture content measuring apparatus is used involumetric titration.
 12. A Karl Fischer's moisture content measuringapparatus according to claim 10, wherein the Karl Fischer's moisturecontent measuring apparatus is used in coulometric titration.
 13. A KarlFischer's moisture content measuring apparatus according to claim 3,wherein the Karl Fischer's moisture content measuring apparatus is usedin volumetric titration.
 14. A Karl Fischer's moisture content measuringapparatus according to claim 3, wherein the Karl Fischer's moisturecontent measuring apparatus is used in coulometric titration.